1. Field of the Invention
The present invention relates to a liquid crystal display apparatus with a wide viewing angle and a method for producing the same.
2. Description of the Related Art
In a liquid crystal display (LCD), a liquid crystal layer including liquid crystal molecules is provided between a pair of substrates. When the alignment of the liquid crystal molecules is changed, the optical refractive index of the liquid crystal layer is also changed. By utilizing the change in the refractive index, the LCD performs the display. Accordingly, it is important that the liquid crystal molecules are arranged as regularly as possible in the initial state. In order to arrange for the regularity of the liquid crystal molecules in the initial state, the surface conditions of the substrates which sandwich the liquid crystal layer should regulate the interactions between the liquid crystal molecules and the surfaces.
In the method for performing such a regulation which is currently the most widely used, materials for liquid crystal alignment films are applied to each of the surfaces of the substrates which face the liquid crystal layer. The applied materials are dried and cured, so as to form the alignment film. Thereafter, the surface of the alignment film is rubbed. When a liquid crystal layer is provided between the substrates having such rubbed alignment films, liquid crystal molecules which are in contact with the alignment films are aligned along the rubbing direction.
There are two types of alignment films for regulating the alignment of the liquid crystal, i.e., an inorganic alignment film and an organic alignment film. Materials for the inorganic alignment film include oxides, inorganic silane, metals, and metal complexes, whereas materials for the organic alignment film include polyimides. A typical material for the liquid crystal alignment film which is currently employed is polyimide resin. The polyimide resin is produced in the following manner. A polyamic acid, which is a precursor for all aromatic polyimides (all aromatic PI), is first applied to a substrate. Then, the substrate with the polyamic acid is heated so that an imidization reaction occurs. As a result, the polyamic acid is converted into a polyimide resin. The reasons why the polyimide resin is widely used for the liquid crystal alignment film material are the concentration and the viscosity thereof can easily be adjusted since the polyamic acid has a good solubility, the polyimide resin has a good applicability, and the thickness of the polyimide resin film can be easily controlled. The produced polyimide resin is more stable in terms of energy than polyamic acid. Accordingly, when the substrate with the polyimide resin is cleaned by water, a reversible reaction will not occur.
The polyimide film which is formed on the substrate in the above-described manner is unidirectionally rubbed with a burnishing cloth or the like. Thus, the liquid crystal molecules can be aligned in the rubbing direction. The rubbing treatment is unidirectionally performed on the substrate, so that the tilt angles (i.e., pretilt angles) of the liquid crystal molecules in the liquid crystal layer which are in contact with the alignment film are all equal to each other. Accordingly, in each picture element constituting a dot as a unit of a matrix-type display pattern, all the pretilt angles are substantially equal to each other and are aligned in one direction.
In an active matrix type LCD which uses thin film transistors as switching elements connected to respective pixel electrodes constituting picture elements of the display pattern, that is, in a TFT-LCD, a construction of a twisted nematic (TN) type liquid crystal layer is adopted (an LCD of the TN mode). In such an LCD of the TN mode, the liquid crystal molecules between the pair of substrates are continuously twisted by 90.degree. along the direction perpendicular to the surfaces of the substrates.
FIG. 20 is a plan view of an exemplary TN type LCD, and FIG. 21A shows a cross section of a picture element portion of the TN type LCD. The LCD is a TFT-LCD of an active matrix type. As is shown in FIG. 21A, a liquid crystal layer 133 is sandwiched between substrates 131 and 132 which are provided so as to face each other. The substrates 131 includes a glass substrate 131a on which scanning lines 112 and signal lines 113 are formed so as to cross each other. In the vicinity of the crossings of the scanning lines 112 and the signal lines 113, thin film transistors (TFTs) 120 as nonlinear switching elements are formed. In areas enclosed by the scanning lines 112 and the signal lines 113, pixel electrodes 110 are formed, respectively, in such a manner that part of each pixel electrode 110 and the scanning line 112 are overlapped. The overlapping area 118 in which the pixel electrode 110 and the scanning line 112 are overlapped functions as an additional capacitance. Each of the TFTs 120 includes a gate electrode 115 which is branched from a scanning line 112, a source electrode 116 which is branched from a signal line 113, and a drain electrode 117 for connecting the TFT 120 to a pixel electrode 110. Over the glass substrate 131a on which the above-mentioned elements are formed, an insulating protective film 131d and an alignment film 131e are formed in that order.
The other substrate 132 also has a glass substrate 132a on which a color filter 132b and a transparent electrode 132c are formed in this order. Over the glass substrate 132a on which the above-mentioned elements are formed, an insulating protective film (not shown) and an alignment film 132e are formed in that order. The alignment film can also function as an insulating protective film.
In the liquid crystal layer 133 sandwiched between the above-described substrates 131 and 132, the liquid crystal molecules are aligned such that the orientation directions are continuously twisted by 90.degree. along the direction perpendicular to the surfaces of the substrates. A liquid crystal molecule 133a near the middle position along the direction perpendicular to the surfaces of the substrates has a predetermined angle with respect to the substrate surface. The substrates 131 and 132 a resealed at their ends by a resin or the like (not shown), and a peripheral circuit or the like for driving the liquid crystal is externally mounted. LCDs which are of types other than the active matrix type also have the same construction as that described above.
In the TN type LCD, by the application of a voltage across the substrates 131 and 132, an electric field is generated in a direction perpendicular to the surfaces of the substrates 131 and 132. In accordance with the dielectric anisotropy of liquid crystal, the liquid crystal molecules stand. By aligning the liquid crystal molecules in parallel to the direction of the electric field, the birefringence of the liquid crystal layer 133 is varied. If the direction of the electric field is perpendicular to the direction to which the liquid crystal molecules stand during no voltage application, that is, if the pretilt angle is 0, the direction to which the liquid crystal molecules stand is not uniquely determined. As a result, a disclination line is generated between liquid crystal domains having different standing directions in response to the electric field. Such a disclination line degrades the display quality. Thus, in order to prevent the generation of the disclination line, as shown in FIG. 21A, the liquid crystal molecules are previously set to be tilted (i.e., to have a pretilt angle).
FIG. 21B shows the initial orientation of liquid crystal when the liquid crystal panel shown in FIG. 21A is viewed from the side of the substrate 132 which is the upper one in FIG. 21A. Vector a in FIG. 21B indicates the rubbing direction of the alignment film 132e, vector b indicates the rubbing direction of the alignment film 131e. The liquid crystal molecules in the vicinity of each of the alignment films 131e and 132e are aligned along the respective rubbing direction (a or b in FIG. 21B) with a pretilt angle .delta.. The rubbing directions a and b forms an angle of 90.degree. therebetween (the twist angle .theta.t=90.degree. in FIG. 21B). The liquid crystal molecules in the liquid crystal layer 133 are continuously twisted by 90.degree. along the thickness direction of the liquid crystal layer 133. Accordingly, the liquid crystal molecule 133a near the middle position in the thickness direction of the liquid crystal layer 133 is also tilted by the angle .delta. with respect to the substrates 131 and 132. The orientation direction of the liquid crystal molecule 133a near the middle position is indicated by vector c in FIG. 21B. The vector c divides the twist angle .theta.t into two equal angles.
Herein, the plus side of the viewing angle .theta.v in FIG. 21A (the side indicated by .theta.1) is referred to as a positive viewing direction, and the minus side of the viewing angle .theta. in FIG. 21A (the side indicated by .theta.2) is referred to as a negative viewing direction. Specifically, the direction in which the liquid crystal panel is viewed from a viewing point on the right side of a broken line in FIG. 21B (i.e., a line which is perpendicular to the orientation direction C of the liquid crystal molecule near the center position of the liquid crystal layer, and which divides the liquid crystal panel into two equal parts) is referred to as a positive viewing direction. The in-plane orientation direction of the liquid crystal panel of the liquid crystal molecule 133a positioned near the center of the liquid crystal layer (c in FIG. 21B) is referred to as a reference orientation direction. As is seen from FIG. 21B, the reference orientation direction divides the twist angle .theta.t of the liquid crystal layer 133 into two equal angles. Also, the minus direction of c is referred to as a reference viewing direction v. That is, the reference viewing direction v is the representative positive viewing direction.
Also herein, an imaginary clockface (dial) is drawn on the liquid crystal panel, and the orientation direction of liquid crystal in the liquid crystal layer is indicated by the clock representation method. Specifically, in the construction in which the display on the liquid crystal panel is actually viewed by a viewer, the upper side of the liquid crystal panel is represented as 12 o'clock, the lower side thereof is represented as 6 o'clock. In a similar way, the orientation direction of the liquid crystal layer is represented as the time in the clock indicated by the reference orientation direction of the liquid crystal layer in the liquid crystal panel. For example, the liquid crystal layer having the reference orientation direction c as shown in FIG. 21B is represented in such a manner as to "be oriented at 3 o'clock" in the construction in which the front side of the figure sheet is regarded as the upper side of the liquid crystal panel.
In the TN mode LCD, since the liquid crystal molecules are aligned in the above-described manner, there occurs a phenomenon in which the contrast is different depending on the angle at which the LCD is viewed. The reasons why the contrast changes will be described below.
FIG. 22 shows an exemplary applied voltage to transmittance characteristics in a normally white mode of an LCD in which light is transmitted during the no voltage application so as to perform a white display.
In FIG. 22, solid line L1 shows the applied voltage to transmittance characteristic when the LCD shown in FIG. 21A is viewed in the direction perpendicular to the surfaces of the substrates (.theta.v=0.degree.). In this case, as the applied voltage value becomes high, the transmittance of light is decreased. When the voltage value reaches a specific value, the transmittance becomes substantially equal to zero. Accordingly, even when a much higher voltage is applied, the transmittance remains at substantially zero.
When the viewing angle is inclined from the direction perpendicular to the substrate face to the positive viewing direction, the applied voltage to transmittance characteristic is varied as is shown by solid line L2 in FIG. 22. Specifically, as the applied voltage becomes high, the transmittance is decreased to some extent. When the applied voltage exceeds a specific value, the transmittance is increased. Then, the transmittance is gradually decreased. Therefore, when the viewing angle is inclined in the positive viewing direction, there occurs a phenomenon in that the black and the white (the negative and positive) of the image are inverted at a specific angle. This phenomenon occurs because the apparent birefringence of liquid crystal molecules having optical anisotropy is varied depending on the viewing angle.
Referring to FIGS. 23A to 23C, the phenomenon will be described in detail. As is shown in FIG. 23A, when the applied voltage is zero or a relatively lower voltage, the center molecule 133a of the liquid crystal layer is observed in the form of an ellipse by the viewer 137 positioned in the positive viewing direction. As the applied voltage is gradually increased, the center molecule 133a is moved in such a manner that the longer axis becomes aligned along the direction of the electric field, i.e., the direction perpendicular to the substrate face. Accordingly, the center molecule 133a is momentarily observed in the form of a circle by the viewer 137, as is shown in FIG. 23B. As the voltage is further increased, the center molecule 133a becomes substantially in parallel to the electric field direction. As a result, the center molecule 133a is observed again in the form of an ellipse by the viewer 137, as is shown in FIG. 23C. In this way, the inversion phenomenon occurs.
If the viewing angle is tilted in the negative viewing direction, the variation of the light transmittance with respect to the applied voltage is relatively small as compared with the case of being viewed from the direction perpendicular to the substrates, as is shown by solid line L3 in FIG. 22. As a result, when the LCD is viewed from the negative viewing direction, the inversion phenomenon does not occur, but the contrast is greatly degraded.
In the TN mode LCD, the inversion phenomenon when viewed from the positive viewing direction and the degradation of contrast when viewed from the negative viewing direction cause serious problems for the viewer, and they result in doubts about the display properties of the LCD.
A technique for suppressing the inversion phenomenon in the TN mode LCD is described in, for example, Japanese Laid-Open Patent Publication No. 2-12. According to the technique, in the active matrix type LCD, a display electrode constituting a picture element is divided into an inner electrode and an outer electrode. By changing the conditions of the electric field applied to the liquid crystal molecules on the inner electrode side from those of the electric field applied to the liquid crystal molecules on the outer electrode side in an attempt to improve the viewing characteristics.
However, the technique disadvantageously necessitates a variety of the electrode patterns, so that the production process and the driving method become complicated. Moreover, the resulting improvement of the viewing angle characteristics is not considered as being so remarkable.
JAPAN DISPLAY '92, pages 591-594, and page 886 describe the following two methods for eliminating the above-described inversion phenomenon. In one method, the surface of the alignment film is unidirectionally rubbed, and then a resist is deposited on a part of the alignment film. Then, the rubbing is performed in the direction reversed to the previous rubbing direction. Thereafter, the resist is removed. As a result, the alignment film is provided with different aligning conditions caused by the different rubbing directions between the alignment film surface covered with the resist and the alignment film surface not covered with the resist, so as to differentiate the pretilt angles. In the other method, alignment films made of different materials are selectively formed and then they are subjected to the rubbing treatment. As a result, a plurality of pretilt angles are formed on the alignment films depending on the materials thereof. By the use of the fact that the reference orientation direction is controlled by the larger one of different pretilt angles, the reference orientation directions which are different from each other by 180.degree. are provided in one and the same liquid crystal layer.
According to these methods, liquid crystal regions having the reference orientation directions different from each other by 180.degree. are mixedly formed in one and the same liquid crystal layer, so that the viewer can mixedly observe the viewing characteristics in both directions. As a result, the inversion phenomenon in the positive viewing direction and also the significant reduction of the contrast in the negative viewing direction are reduced and improved. However, these methods include a photolithography process, and hence involve a contamination problem of the alignment films. In the former proposed method, if the resist is deposited on the surface of the alignment film, the alignment regulating property of the alignment film surface is greatly deteriorated. In the latter proposed method, the patterning of the alignment films requires complicated processing steps. For these reasons, the above methods are not practical.
In another attempt to eliminate the inversion phenomenon in the positive viewing direction and the contrast degradation in the negative viewing direction, a rectangular region 119 in which the reference orientation direction is different from that in the other region is formed in part of a picture element shown by a dotted line in FIG. 19. In more detail, the liquid crystal regions having the reference orientation directions which are different from each other by 180.degree. are formed in one picture element, so that the contrast degradation in the negative viewing direction is compensated, and the inversion phenomenon in the positive viewing direction is suppressed.
However, in the above method in which liquid crystal regions having the reference orientation directions which are different from each other in one picture element, as the time elapses, the aligning condition of one region may be absorbed by the aligning condition of the other region. In addition, in the boundary area between the liquid crystal regions (an area indicated by dimension line in FIG. 21A), a disclination line occurs, i.e., the liquid crystals cannot be driven by the influence of both aligning conditions. This causes the contrast to be degraded.